Electrical measuring and regulat



March 9, 1937. H. L. BERNARDE 20,282

ELECTRICAL MEASURING AND REGULATING SYSTEM Original Filed July 1, 1 932 3 Sheets-Sheet 3 fig. 5

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77 v BY V zv'h ATTOR Y Reissued Mar. 9, 1937:

"UNITED STATES ELECTRICAL MEASURING AND REGULAT- ING SYSTEM Henry L. Bernarde, Kearny,

Westinghouse Electric &' Mann! N. 1., assignmto aeturingCom pany, East Pittsburgh, Pa., a corporation of Pennsylvania Original No. Serial No.

division of Serial No. isazpgigation for reissue June 5, 1936,

15 Claims.

My invention relates to electronic recorders or regulators ,and more specifically to systems wherein a delicate measuring instrument, such as a galvanometer is arranged to control a comparatively heavy indicating or recording instru ment, or a control device such as a valve, switch, or rheostat.

In previous systems of this nature it has been necessary to use a Kelvin balance or other balm anclng system involving the use of numerous mechanically moving parts and electrical contacts, thereby introducing various complications and errors. Other difliculties experienced with previous systems are slow-operation, mechanical friction and the tendency to overrun and hunt. It is accordingly an object of my invention to provide 'an accurate recording system which isquicl: in its action and does not overrun or hunt, and which may be applied to the most sensitive mechanism.

A further object of my invention is to provide a recording system whichds small, cheap and simple. and which is not subject to inaccuracies due to mechanical load and friction.

Another object of my invention is to provide an electronic system wherein a feeble alternating voltage derived by the movement of a delicate instrument, such as a galvanometer, is amplified sufllcientlyto control relatively heavy apparatus. 30 In accordance with my invention the movement of'a galvanometer controls means for deriving from an original alternating voltage a derived alternating voltage variable ln phase and amplitude. In order to avoid interference with the delicate rotatable element of the galvanometer, various space effects are utilized for this purpose. I

In one embodiment of my invention the voltage deriving means comprises a shutter which is 0 moved by the galvanometer to control the light failing upon a photo-cell from a pair of alternately flashing lamps energized from the original alternating current source on alternate half cycles. The electrical impulses set up by the 45 light flashes on the photo-cell will be in phase with one or the other of the half cycles of the original alternating current depending upon which beam oi light is permitted to fall upon the cell. and its magnitude will vary according to the amount of light passed. In another embodiment of my invention a synchronous lightchopper is used with a steady source of light instead of the flashing lamps. Other forms of 55 modulated light sources obvlouslymay be used.

Gal

1,973,279, dated September 11, 1934, 667,346, April 22,

1933, which is I 620387, July 1, 1932. Serial No.

Such light controlled systems are fully disclosed in my copending application Serial No. 620,387, filed July 1, 1932 and from which the present application has been divided.

In the embodiment herein disclosed the galva-.

nometer carries a very small pick-up coil which it rotates in an alternating fleld set up by the original alternating voltage. When the pick-up coil is turned in one direction from its zero position, the alternating voltage induced therein is 180 out of phase with reference to the voltage which would be induced therein if it were turned in the opposite direction from its zero position. Zero position is that position of the pick-up coil where the induced voltage is actually zero.

Although the voltages set up by either voltage deriving means may be very feeble, these voltages may be suflicientiy amplified by a suitable amplifier that they may be used for actuating comparatively heavy apparatus.

When the apparatus to be actuated comprises a recording or indicating element it is necessary to provide suitable arresting means for inter-,

rupting the motion of the element when it has been moved to a position which corresponds to the condition or quantity which controls the galvanometer. Otherwise the actuated element would continue to move to its extreme position. This is readily accomplished by providing a second pick-up coil on the controlled element. The second coil is connected in series-opposing relation to the first coil, and as it is rotated by the controlled element a position is reached wherein its voltage is exactly equal and opposite to that induced in the first pick-up coil. The voltage of the first pick-up coil being exactly balanced.

the controlled element comes to rest and remains stationary until this state of equilibrium is disturbed by another movement of the first coil.

Another arrangement employed for arresting the movement of the controlled element introduces a counter-potential into the measuring clr-' cult to thereby restore the galvanometer to zero. This may be accomplished by a potentiometer, the

slidewire contact of which is connected to the controlled element for movement therewith. A simpler method, avoiding the use or a potentiometer with its sliding contact, consists in passing the controlling current through a. resistor in the galvanometer circuit in such a direction that the IR drop opposes the measuring current.

when the controlled element is only moved between two extreme positions as a switch or a valve, it isunnecessary to discontinue the actuating force in any intermediate position and the arresting means referred to above may be omitted. thereby still further simplifying the system tional voltage is accomplished by a small pickup coil rotated by the galvanometer in an allfi-ternating field set up by the original potential;

Fig. 2 is a diagrammatic view showing a recording system wherein a second pick-up coil is rotated by the pen to provide a balancing voltage which is equal and opposed to that induced in the first pick-up coil when it has been moved to the proper position;

Fig. 3 is a diagrammatic view showing how a graphic recorder actuated by such a system may control a slide wire potentiometer to interpose a counterpotential in the measuring circuit of the galvanometer for balancing it back to zero when the pen has been moved to its proper po-' sition:

Fig. 4 is a diagrammatic view showing a system provided with a special amplifier for actuating a graphic milliammeter of the DArsonval type which is so connected that its controlling current also passes directly through a suitable resistor in the measuring circuit of the galvanometer for balancing it; and

Fig. 5 is a diagrammatic view showing an amplifier and source of current suitable for use in such a system.

In the drawings similar reference characters have been applied throughout to similar elements in the various systems and new reference characters have been applied only to new or modified elements.

Referring more specifically to the drawings,

the apparatus shown in Fig. 1 comprises a system wherein electromagnetic induction is utilized by-mounting a small pick-up coil I I I on the moving element II2 of the galvanometer, voltmeter,

, pressure meter or any other meter which is reso sponsive to a quantity or condition which is to be controlled. A suitable field structure m, which is energized from .the alternating current line Ll L2, is so positioned relative to the pickup coil III that the latter will be in zero inductive relation thereto when the condition to be' maintained is at it correct value. c

When the condition varies, the meter II2 rotates the pick-up coil III into inductive relation in the field. By connecting the pick-up coll so ill through suitable pig tall or slip ring connections Ill and conductors H8 to the input of a suitable amplifier tube Ill the induced voltages may be amplified without drawing any appreciable current from the coil. It is therefore possible to utilize a very small pick-up coil which does not interfere with the operation of the meter. I

The filament of the tube Ill may be energized from the secondary winding 88 of a transformer T having a center tap B9 by which it is connected through a biasing resistor R1 to the conductor IIO, and-the negative terminal of the plate battery ll.

- The output of the amplifier is coupled to one a coil III of an induction disk motor through any -phase relation of the tor.

suitable coupling such as impedance coil Ill and condenser III whereby the amplified potentials are impressed thereon. A second coil I2. of the induction disk motor is connected directly to the alternating current line, from which it is continuously excited. and when both coils areenergized the simultaneous on of the two windings creates a torque in disk as thereby causing it to rotate.

The direction of the torque reacting on the disk OI of the motor'depends upon the relative alternating currents traversing the coils. which in turn depends upon the inductive relation of the pick-upcoil 'in its surrounding field. The phase relation of the induced potentials varies J degrees depending upon which direction the pick-up coil is turned from its zero inductive position which thereby determines the direction of rotation of the mo- The magnitude of the induced potentials, and hence the speed of the motor, varies according to the amount the pick-up coil is rotated from its zero position. Hence both the direction and the speed of rotation of the motor is controlled by the rotation of the small pickup coilon the meter, and it may actuate rheostats, valves, relays or other control elements for controlling the condition to be maintained. A commutator type motor may be substituted for the disk induction motor if desired.

when it is desired to utilize the motor for driving a penlor marker to make a record on a chart, it is necessary'to provide some means for arresting the motion of the motor when the movement of the marker has been proportional to the variable condition which initiated the operation.

The actuation of a. recorder is shown in Fig. 2 wherein the motor disk is connected through suitable gears I2I, I22 to rotate a second shaft I23 and move a pen I24 over a chart I25. A second pick-up coil |2I.also is mounted on the second shaft I23 of the motor. Disposed in inductive relation adjacent thereto is a field structure I21 which is excited from the A. 0. line. The second pick-up coil I26 is connected in a series circuit I28, I28, I" in opposed relation with the first pick-up coil Ill, and the resulting potential which is impressed on the input of the amplifier is the diiference, or algebraic sum of the two. The pick-up coils and their associated field structures may be similar so that their induced potentials will be equal and the potential impressed on the amplifier will be zero when the coils are rotated to equal angular positions.

Hence, any rotation of the first coil III by the meter II2 will cause the motor to run, but as the second coil I26 approaches a position which corresponds to that of the first coil the induced potentials will gradually approach equality and the motor disk 99 will gradually come to rest when the pen I24 reaches the correct position on the chart. Such arresting means is equally applicable to systems which are controlled either by electrical or non-electrical meters, and since the motor is brought to a gradual stop, the system will not overrun and hunt. Furthermore, when the galvanometer is deflected a large amount, and the difference between the voltages induced in the pick-up coils III and I28 is large,

byaitsnatingcurrentofahigherfrequsncy intheamplifier.atthepick-uppoint,oreven whichmaybesuppliedbyatubeoscillator, or other means. I

It willbeunderstoodthat additional stages of 5 amplification in be provided with suitable .power tubes to supply. the amount of output powerrequired. Ibrexamplaforthispurposea very satisfactory amusement in the amplifier comprises a high mu as type tube the control grid of which is connected to the pick-up coil, and the plate circuit of which is coupled to the grid of a power tube. preferably of-the. pentode m- I I I 4 Referring to Fig. 3, instead of determiningthe position of the pen by balancing o the alternating induced voltages in the -up coil by voltaga induced in a-second pick-up coil. it .is. possible when the measuring meter is an elec-.

trical instrument such as a salvanometer, to balance the current in the galvanometer measuring circuit and thereby restore the 'galvanometer and the pick-up coil to zero position when the marker has moved to its proper position. This may be readily accomplished by so connecting the slide wire resistor ill of a pot'entiome ter III into the galvanometer input circuit in series with the thermocouple Ill that its IR drop will oppose the measuringvoltage from the thermocouple III or other controlling element which influences the galvanometer I40.

In this arrangement a .threaded shaft I is driven by a motor and adjusts the movable brush I of the potentiometer accordingly as it simultaneously moves the pen I45 across the chart I 40. The potentiometer is manually so adjusted by a variable resistor I" that the counter-potential interposed in the galvanometer circuit will be equal to the potential impressed thereon by the thermocouple lit, or other controlling 40 element, when the pen I" has been moved to its proper position on the chart. The galvanometer I will then return to its zero position thereby turning the small pick-up coil ill to its zero inductive position and stopping the mo tor 9Q.

Whenever the galvanometer turns from its zero position, potentials are induced in the pickup coil Ill which cause the motor disk 00 to run in one direction .or the other. depending upon the direction in which the pick-up coil was rotated. As the pen is moved across the chart, the potentiometer brush I is simultaneously moved in the same direction whereby it gradually varies the counter-potential interposed by the potentiometer in the proper direction to gradually balance outthe measuring potential impressed on the 'galvan'ometer circuit by the thermocouple ll. or other measuring device. The galvanometer Ill gradually returns to its zero position and the motor disk 09 gradually comes to rest when the pen I45 has been moved I to its proper position. Since the arresting means is effective to stop the motor gradually, the system will not overrun and hunt but is stable in operation.

In the use of two phase devices such as the induction disk motor or the relays utilized in my system, the maximum torque will be produced when the fluxes produced by the two sets of coils in the device are 90 degrees apart. This phase relation may be adjusted anywhere in the system by the use of condensers, inductances andreslstancesintheusualmanner. Thephase shifting apparatus may be inserted at some point in the motor coils themselves.

I have found'lt possible to entirely eliminate the potentiometer with its battery and-sliding brush attached to the pen and to directly balance out the current in. the .galvanometer circuit by feeding bachthe current from the circuit of the graphic recording instrument. The preferred embodiment of such a system is diagrammatically represented in Fig. 4.

.The output of the first amplifier tube I It in this sy tem feeds into a full wave rectifier ill the plate I of the tube. The other terminal of the primary winding is connected to the oathode llt through a suitable B battery III, or source of current for plate. The secondary winding I51 of the former is connected to the rectifier iii in series relation with the secondary winding III of a second transformer I59 which has its primary winding connected directiv to the 60 cycle A. 0. line conductors Ll L2. The secondary winding of the second transformer is designed to generate about 25 volts. The movement of the galvanometer pick-up coil III is so limited by stops. or in any suitable manner, that in its position of maximum angular deflection the output for the secondary I51 of the first transformer III is about 25 volts.

The full wave rectifier Iii may be made up of copper oxide elements and its direct current output is applied to the grid and cathode oi a second vacuum tube I which may be a 245 power tube. A resistor R0 of about 1 megohm is inserted in the grid connection and a condenser I of about 4 mfd. capacity is connected be tween the grid and the cathode of the tube. A resistor R0 is shunted across the output terminals of the rectifier, and may have a resistance of about 50,000 ohms. A circuit extends from the plate of the tube I through a resistor RM and suitable B battery I00 or other source of current to the cathode. The 13 source is shunted by resistors RH and Bit, and from the intermediate point C connection is made with conductor I51 of the output circuit. The other side of the output circuit comprises a conductor I08 which joins the plate circuit at a point D between the plate and the resistor RIO. Said output conductors are connected to a directcurrent type graphic recorder I'll through a resistor Rlt. The resistor Rll is included in galvanometer input circuit in series with the thermocouple 2|! or other measuring device.

A specific operation of the system may be traced as follows. Assume the temperature difference between the two ends of the thermocouple III to be suiilclent to generate .015 volt, and also that the resistor Rll in series with the couple has a resistance of 10 ohms. To balance the .015 volt generated by the couple, a current of .0015 ampere must fiow through the resistor Rlt.

Suppose for-a moment that the voltage across the 4 mid. condenser I65, and on the grid of the associated tube I. is such a value as to allow a current of the correct value of .0015 ampere to flow through the 10 ohm resistor Rll. Under thesev conditions the galvanometer will be balanced and the pick-up coil will be at its zero position. But when the pick-upcoil is at zero, the input to the rectifier is 25 volts. with this input the D. C. output voltage from the rectifier across the 4 mfd. condenser I05 will start to the resistors Itil. Bil and all associated with this Double."

dthelasttubeisadiustedtomake 'lhemomentthecurrentthroughthemohm resistor Rll drops slightly below .0015 ampere.

the voltageaeross this resistor no longer balances the voltage generated by the thermocou- Because of the above condition unbalanced voltages, the galvanometer Ill and also the pickup coil ill changes the input tothe rectifier. which causes the current through the resistor 15 ml to slowly (because of the time delay calmed by the condenser ill being charged in series with the 1 megohm resistor R8) increase to a value slightly above the correct value oi .0016 ampere, which again unbalances the galvanom- 20 eter and pick-up coil, but this time the unbalanceisinsuchadirectionastcbringthecurrent slightly below the correct value of .0015

ampere.

At all times, except when the thermocouple 2 ill is generating aero voltage, the galvanometer III is oscillating a small amount and also the current through the resistor Rll and the recording milliammeter "i is varying slightly above and below the value necessary to balance the 80 voltage of the couple. The varying current moves the milliammeter only slightly, probably lesathan the width of a mark made by the pen Ill, because of the greater sensitivity or the galvanometer over the. milliammeter.

35 In Fig. 5 I have shown an amplifier suitable for use in the above systems and a power supply whereby the amplifier may be directly from the 110 volt 00 cycle A. C. mains. As in the systems disclosed above, a field struc- 40 ture ill is excited directly ,irom the line conductors Ll LI and has associated therewith a pick-up coll III which may be mounted on the rotatable element 01 a galvanometer, a pressure gauge, or a meter responsive to any condition to 45 be observed or regulated. The output from the pick-up coil is impressed upon the first tube ill of the amplifier which is preferably a 22"! high mu tube. One terminal of the coil III is connected directly to the grid oi! the tube ill through 60 a conductor ill and the other terminal is connected to the cathode through a conductor ill and a pair of resistors RI! and Bil of 100,000

' and 750 ohms resistance respectively connected in series and shunted by a condenser ill oi about as .25 mid. capacity.

- Theoutputorplatecircuitoithefirsttube ill extends by a conductor ill through a coupling impedance ll'l oi about 200 henrles and a resistor R." of about 20,000 ohms. The plate 00 end of the circuit is coupled from the conductor llltothegridoithesecondtube ill through a condenser ill of about .1 mid. capacity. The second tube III is preferably a 247 tube and its filament is shunted by a resistor Bil of ohms '65 which is center tapped to provide a constant potential point E for connection with the grid returncircuitandthecathodeotthefirsttube ll. An interconnecting conductor Ill extends iromthecentertapliottheshuntingresistor 70R" throughabiasingrsistor-Rlltoapoint 1'' between the two resistors Rllsnd Bil associatedwiththecathodeoithefirsttubelli. The gridoithesecondtube Ill isconnectedtothe cathode circuitconductor Ill througharesistor rsnllolsboutlwmohmsresistance. Theplate or output circuit crthe second tube extends y wayoiaconductor lllthroughonecoil Ill otatwophasedevicetobeactmtedthereby. thence to the screen grid of the second tube and theplatecircuitotthefirsttub'ebywayola s conductor Ill. r

A power transformer III is directly energized iromtheA.C.lineconductorsLlI.-2andaplurality 01 secondary windings SI, 8! and Bl afiord suitable volta es for enemifl s various 10 elements oi the amplifier. One lowvoltage secondary winding Si is connected by suitable conductors ill, Ill directly to the filaments of the two amplifier tubes ill. Ill which are connected in parallel by suitable conductors Ill, 2" wherebybothar eenergisedbyalternatingcurrentoi a proper voltage.

Inorder thatsuitabledirectcurrentmaybe supplied to the plates 0! the amplifier tubes a i'ullwavetubelllsuchssthetypemisprovided and its filament is energised by alternatingcurrentotapropervoltagesuppliedbya suitable secondary winding 82 on the power transformer. A center tapped high voltage secondary winding Bl has its-terminals connected to the respective plates of the rectifier tube ill, and a conductor lll extends tram the center tap G to the interconnected cathode conductor Ill of the amplifier tubes. From the filament oi the rectifier tube a circuit extends to the plate circuits of the amplifier tubes through a conductor ill, a resistor R2! oi! about 1000 ohms, an impedance coil Ill 0! about 20 henrles inductance. and a conductor Ill which Joins the conductor Ill. Large filter condensers Ill and ill of about 8 mid. capacity each are' connected between the transiormer center tap conductor 201 and the respective terminals of the impedance coil Ill.

The coil of the actuated device in the plate ciro cult of the last amplifier tube is shunted by a condenser 2N oi .5 capacity. The other coil ll! of the controlled device is connected directly to the main A. C. line conductors Ll Ll through a circuit comprising conductors Ill and 2", having a resistor R2! of about75 ohms in series therewith to obtain the most efiective phase relsTtion between the currents in the two coils ill and ill for maximum torque.

The measuring device on which the pick-up cell III is mounted rotates the latter in one direction or the other in accordance .with variations of the influencing condition. As the voltages induced in the pick-up coil iii are impressed on the amplifier they are amplified in magnitude and excitethecoilo! the controlled device which isconnectedintheplatecircuitoithelastamplifiertube. Thedeviceisthusactuatedina direction which is determined by the relative phase relation of the alternating currents in the twowindingsoithedeviceandataspeedwhich is determined by the magnitude or the A. C. voltages induced in the pick-up coil, both oi. which depend upon the direction and degree of rotation of the latter. 1

AlthoughI have shown separate sources or current ioreachplateandfilamentintheamplifiersofmanyottheabovesystemsandhave representedthemasbatteries,thishasbeendone merelyt'osimpliiythediagramsanditistobe understoodthatthesecircuitsmaybecnergised from a B-battery eliminator as shown above Fig. 6 or in any suitable manner. 'Itwillbeseen.thereiore,thatlhaveprovided 'anelectronicsystemutilisingvariomspaeeei- 75 90 various other modifications thereof may be eftected without departing from the spirit of my invention; My invention, therefore, is not to be limited except as necessitated by the prior art and the scope of the appended claims.

I claim as my invention:

1. In an apparatus for measuring a variable condition, a source 01 alternating voltage, movable means ior indicating the magnitude of said condition, operating means for said movable meanscomprising a primary measuring element,

a variable impedance device energized from said source and controlled by said primary element, an electronic device controlled in accordance with an electrical condition derived from said vari- 5 time impedance device, and motive means responsive to an output condition of said electronic device for producing movement 01' said movable means in accordance with said variable condition, and follow-up means. for controlling said operating means to arrest movement of said movable means when the deflection thereof corresponds to the magnitude of said variable condition.

2. In an apparatus for measuring a variable condition, a source of alternating voltage, movable means ipr indicating the magnitude of said condition, operating means for said movable means comprising a primary measuring element,

a variable impedance device energized from said source and controlled by said primary element,

an electronic device energized in accordance with an electrical condition derived from said variable impedance device, and an alternating-current motor for producing movement 01' said movable means in accordance with said variable condition, said motorhavinga winding energized from said source and a second winding responsive to an output condition oi said electronic device, and follow-up means for controlling said operating means to arrest movement 01' said movable means when the deflection thereof corresponds to the magnitude of said variable condition.

3. In an apparatus for measuring a variable condition, a source of alternating voltage, movable means for indicating the magnitude of said condition, operating means for said movable means comprising a primary measuring element, a variable mutual inductance device energized from said source and controlled by said primary element, an electronic amplifier energized from said source through said variable mutual inductance device, ,and an alternating-current motor for producing movement of said movable means in accordance with said variable condition, said 75 motor having a winding energized from said voltages, frequencies, inductance. capacities and resistances therein set forth since sourceandasecondwindingenergizedinaccordance with the alterna current output of said amplifier, and follow-up means for controlling said operating means to arrest movement of said movable means when the deflection thereof corfuespont1 ds to the magnitude of said variable con- 4. In an apparatus responsive to a variable condition, a source of alternating voltage, a primary measuring element, an inductive device comprising means energized from said source for producing an alternating held, and a movable ,pick-up coil in variable inductive relation to said iield, said pick-up coil being controlled by said primary element, an electronic device responsive to the voltage induced in said pick-up coil, and a translating device energized in accordance with an output condition 01 said electronic device.

5. In an apparatus responsive to a variablecondition, a sauce of alternative voltage, a primary measuring element, an inductive device comprising a magnetic structure having an airgap therein, an energizing winding connected to said source, said energizing winding being inductively associated with said magnetic structure, and a pick-up coil mounted for movement in said air gap, said pick-up coil being controlled by said primary element, an electronic device responsive to the voltage induced in said pick-up coil, and a translating device energized in accordance with an output condition of said electronic device.

6. In an apparatus for measuring a variable condition, a source of alternating voltage, movable means for indicating the magnitude of said condition, operating means for said movable means comprising a primary measuring element, an inductive device including a magnetic structure having an air gap therein, an energizing winding connected to said source, said energizing winding being inductively associated with said magnetic structure, and a pick-up coil mounted for movement in said air gap, said pick-up coil being controlled by said primary element, an electronic device controlled in accordance with an electrical condition derived from said inductive device, and motive means responsive to an output condition pi said-electronic device for producing movement oi said movable means in accordance with said condition, and follow-up means for controlling said operating means to arrest movement of said movable means when the deflection thereof corresponds to the magnitude of said variable condition.

7. In an apparatus for measuring a variable condition, a source of alternating voltage, movable means for indicating the magnitude of said condition, operating means for said movable means comprising a primary measuring element, an inductive device including a magnetic structure having an airgap therein, an energizing winding connected to said source, said energizing winding beir'ig inductively associated with said magnetic structure, and a pick-up coil mounted for movement in said air gap, said pick-up coil being controlled by said primary element, an electronic amplifier energized in accordance with the alternating voltage induced in said pick-up coil, and an alternating current motor for producing movement of said movable means in accordance with-said condition, said motor having a winding energized from said source and a second winding energized in accordance with the alternating current output of said amplifier, and follow-up means for controlling said operating said variable impedanecdevice, motive means responsive to an output condition of said electronic device for producing t of said stylus in accordance with said variable condition. and follow-up means comprising a second variable device similar to said first-mentioned sition of said stylus and being electrically connected to produce an output condition opposing said output condition of said first-mentioned variable impedance device. '9.Inan apparatusiorrecordingavariable condition, a source of alternating voltage, a recording stylus, a primary measuring element, an

inductive device including a magnetic structure having an air gap therein,-an winding connected to said soin'ce, saidenergizlng winding being inductively associated with said magnetic structure, and a pick-up coil mounted for movement in said air 88!). said pick-up coil being controlled by said primary element, an electronic device controlled in with an electrical condition derived from said inductive device, motive means responsive to an output condition of said electronic device for producing movement of said stylus in accordance with said condition,

- and follow-up means comprising a second inductive device including a second magnetic structure having a second air gap therein, a second energizing winding connected to said source,said

second energizing winding being inductively associated with said second magnetic structure and a second pick-up coil mounted for movement in saidsecond air gap, said second pick-up coil bebeing connected electrically in opposition to said first-mentioned pick-up coil.

10. In combination means for deriving from an original source oi alternating voltage an additlonal alternating voltage variable in instantaneous polarity and amplitude according to a controlling quantity or condition comprising means energized from said original source oi alternating voltage for setting up an alternating field, a pick-up coil in said field, means for rotating said coil to Iero inductive position or either direction therefrom accordingly as said quantity or condition varies, and means responsive to the simultaneous eiiects of said derived and original voltages, a second pick-up coil disposed in a field set up by said original source of alternating voltage, a circuit including said responsive means in series with said pick-up coils in opposing relation, and means associating said second pick-up coil with said responsive means whereby the actuation oi the latter will vary the inductive relation oi the coil. in the field to set up a balancing counter voltage when the actuation has been proportional to said controlling quantity or condition.

11. In combination means for deriving from an original source of alternating voltage an additional alternating voltage variable in instantaneous polarity and amplitude according to a 75 variable quantity or condition comprising, means ing mounted for movement with said stylus and thecoiltotheiieldisvariedinaccordancewith the actuation oi the galvanometer an electroresponsive device actuated by the simultaneous ei'iect of the voltages induced in said pick-1P coil and said original voltage. a segand pick-up coil actuated by the electro-responsive device in inductiverelationtoafield setupbytheoriginai voltage source, and means 'i'or connecting the electro-responsive device in series with said coils 'in opposing relation whereby the actuation 01-- said electro-responsive device causes the second coil to induce an opp s ng alternating voltage which balances the voltage-induced in the first pick-up coil when the device has been actuated an amount proportional to the amount the gal- Wanometer has been actuated by said variable quantity or condition.

12. In combination, adirect current type electro-responsive device, means for energizing saiddevice by direct current variable in accordance with some quantity or condition, responsive means set into operation thereby, arresting means for interrupting the operation of said responsive means when its actuation is proportional to the variation of said quantity or condition comprising means for introducing a counter voltage in w the energizing current circuit.

13. In combination means for deriving from an original source 01' alternating voltage an'additional voltage variable in instantaneous polarity and amplitude relative thereto, means for controlling said voltage deriving means comprising an electro-responsive device energized by a measuring current variable in accordance with some quantity or condition, responsive means actuated by the simultaneous eil'eet of said original and said derived voltages, arresting means. for interrupting the actuation of said responsive means when its actuation is proportional to the variation of said condition or quantity comprising means driven by the responsive means for introducing a counter potential into the measuring current circuit to restore the device and the associated voltage deriving means to zero position. I 1

14. In combination means for deriving from an original source of alternating voltage an additional voltage variable in instantaneous polarity and amplitude in accordance with a variable quantity or condition comprising means energized by said original voltage source for setting up an alternating field, a pick-up coil disposed in inductive relation to said field, means for varying the inductive relation oi said coil in said field in accordance with variations of said quantity or condition, an amplifier, means connecting said pick-up coil to the input oi said amplifier, a twoand amplitude in accordance with a variable,

quantity or condition comprising means energised by said original source of alternating voltage for setting up an alternating field, a pick-up some: -7

coil dispooed ill-inductive relation to aald'fleld, means for name the inductive relation of said oollineaidfleldin with variation;

d quantity or condition, an amplifier comahighmuamplinertube.agrldoontrolled reotmertube. means for the plateoinldrectiflermbeiromlaidorlginel source of alternating voltage. a power output tube suitably coupled thereto. and a graphic millimmeterintheplatecircuitofthelasttube.

HENRY L. BERNARDI. 

